Santiaguito is an open vent volcano. It contains magma-air interfaces through which gas passes. It is important to study the gas emissions from open vent volcanoes because they represent the simplest case of gas-magma interactions. Open vent volcanoes do not have the added complexities introduced by gas passing through a fractured volcanic edifice through which numerous reactions could result (Andres et al., 1993).

Santiaguito has been erupting a hi-Na, calc-alkalic dacite since it began to grow in 1922 (Rose, 1987b). Rose (1987b) shows a cyclicity in dome extrusion rates since 1922, in which 3-5 year high extrusion rates alternate with muc hlower rates. A seventh period of increased dome extrusion began in 1986. Microprobe analyses of melt inclusion and interstitial glass in a 1902 Santa María dacite pumice (compositionally similar to Santiaguito lavas) indicate pre- and post-eruptive S contents which average 200 and 110 ppm S, respectively (Palais and Sigurdsson, 1989).

Santiaguito shows a general pattern of greater SO2 emission rates during eruptions than between eruptions (fuming). During vertical ash eruptions, which happen between 5 and 25% of the time each day, SO2 emission rates may rise to several thousand Mg/d. Between these eruptions, SO2 emission rates vary between 20 and 420 Mg/d. Considering relatively short-lived eruptive events and lengthy fuming periods, the daily Santiaguito SO2 emission rate is approximately 50 to 100 Mg/day. (Andres et al., 1993)

SO2 activity plot for Santiaguito. SO2 averages (solid) and ranges (stippled) are shown. The occurrence of various styles of activity are portrayed by the corresponding activity bars. During this period for Santiaguito, dome growth occurred primarily through lava flows being extruded on its surface (fast lava flows) with little endogenous growth, In the middle of the two-decade period, the extrusion rate of the lava flows decreased significantly (slow lava flows)(Andres et al., 1993).

(Andres et al., 1993)